If you run Calibre in DEBUG mode, it will show you exactly what codes it got from where.
FAST codes are gotten along with DDC and LCC from the official OCLC Classify API.
LCEAD codes are "keywords" gotten from WorldCat via LC-specific webscraping.
There does appear to be an overlap (they pertain to the same book after all), with some books having more common tags and others having fewer. However, they are usually not identical, especially for Non-Fiction.
Code:
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book: 71
oclc: 1077784
final loc_lccn_dict: {}
new value for LC Extra Author Data: Einsteinˏ Albert : 1879-1955
new value for LC Extra Author Data: Harrisˏ Alan [Translator; Creator]
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edit_metadata_action.apply_metadata_changes(id_map, callback=self.finish_displaying_results_of_ddc_lcc(payload)):
id_map - book: 71
id_map - mi: Title : Unknown
Author(s) : Unknown
Tags : Factual:People-Albert Einstein, Factual:Science-Physics
Identifiers : viaf_author_id:75121530, lc_authority_name:n79022889, isbn:9781453204948, isni:000000012281955, oclc-owi:3754588294, oclc:1077784, lccn:lccn-n79022889
FAST Tags : Jews, Peace, Physics, Science--Philosophy, Science, Einstein, Albert, 1879-1955, Science--Social aspects, Religion and science
DDC : 081
LCC : AC35
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